A non-oriented electromagnetic steel sheet of grade S9, which is presently considered to be of the highest grade, is relatively frequently used as the magnetic core material for large-sized rotary machines and the like. High-grade non-oriented electromagnetic steel sheets exhibit a low watt loss, but the magnetic flux density thereof is poor. For this reason, electric companies do not always use high-grade non-oriented electromagnetic steel sheets as the magnetic core material for large-sized rotary machines but instead use grain-oriented silicon steel sheets which have a high magnetic flux density and are expensive.
Recently, it has been considered whether a high-grade non-oriented electromagnetic steel sheet having qualities such that the sheet can be satisfactorily used as the magnetic core material for large-sized rotary machines, while recently the aim being to conserve energy and reduce the cost of production. Therefore, there has been a great demand for a higher-grade non-oriented electromagnetic steel sheet having a lower watt loss and an improved magnetic flux density as compared with the grade S9 non-oriented electromagnetic steel sheet.
The watt loss of a non-oriented electromagnetic steel sheet can be reduced by increasing the silicon or aluminum content in the sheet and by increasing the size of the crystal grains of the product. However, increasing the silicon or aluminum content and increasing the size of the crystal grains by, for example, enhancing the finishing-annealing temperature of a steel strip cause the resultant product to exhibit a decreased magnetic flux density.
Several proposals have been made for producing a high-grade non-oriented electromagnetic steel sheet of grade S7 or grade S8. For example, Japanese Laid-open Patent Application No. 53-66816 discloses a process for producing a non-oriented electromagnetic steel sheet including a so-called double-stage cold-rolling in which a hot-rolled steel sheet is subjected to cold-rolling twice and to intermediate annealing between the first and second cold-rolling steps. In accordance with this process, the sulfur content and the oxygen content in a silicon steel are restricted to trace levels of 0.005% or less and 0.0025% or less, respectively, so as to suppress the formation of fine inclusions in the steel, thereby not hindering crystal growth during annealing; the sheet is subjected to intermediate annealing at a temperature of from 900.degree. C. to 1050.degree. C. for a relatively long period of, for example, from 2 to 15 minutes so as to form large crystal grains having an average grain diameter of 0.07 mm or more; and, the resultant steel having an intermediate sheet thickness is subjected to cold-rolling and is subjected finally to finishing-annealing at a temperature of from 930.degree. C. to 1050.degree. C. for a sufficient period of from 2 to 15 minutes so that the crystal grains have orientations which are desirable for the magnetic flux density of the sheet, thereby improving the magnetic properties of the final product.
However, since this process requires a relatively long period of time (2 to 15 minutes) for both intermediate annealing and final finishing-annealing, internal oxidation of the steel sheet is likely to occur due to the annealing atmosphere, possibly resulting in deterioration of the magnetic properties of the final product. Since internal oxidation of the steel sheet is likely to occur particularly in the final finishing-annealing step, a long annealing period promotes deterioration of the magnetic properties of the final product. Adopting the size of the crystal grains of an intermediate-annealed steel having an intermediate sheet thickness as a criterion in the manufacture of the sheet involves a problem in producing a final product having stable magnetic properties because the crystal grain size cannot be immediately determined during manufacture of the sheet. In addition, this type of criterion makes it impossible to increase the production speed.
Japanese Laid-open Patent Application No. 55-97426 discloses a process for producing a non-oriented electromagnetic steel sheet including single-stage cold-rolling. In accordance with this process, the sulfur content and the nitrogen content is a silicon steel are restricted to not more than 0.005% and not more than 0.004%, respectively, so as to suppress the formation of fine inclusions and precipitates, thereby improving the magnetic properties of the final product; and, in order to prevent internal oxidation of the steel sheet, the hot-rolled steel is subjected to annealing in a non-decarburizing atmosphere and the resultant steel sheet is subjected to finishing-annealing in a non-oxidizing atmosphere or in a decarburizing atmosphere after being coated with a solution of an alkali metal salt at a temperature of from 950.degree. C. to 1100.degree. C. for a period of from 1 to 5 minutes, thereby preventing internal oxidation and thus improving the magnetic properties of the final product. However, it is difficult to produce by means of this process a high-grade non-oriented electromagnetic steel sheet having a stable quality of grade S7 or grade S 8, and a non-oriented electromagnetic steel sheet of grade S7 or grade S8 having a stable quality has not yet been produced.